The present invention relates to methods and systems for regulating power in an electricity grid system.
As is well known in the art an AC electricity grid system is organized as a grid or network that interconnects multiple power generating facilities with multiple loads. The grid is typically geographically diverse and is subdivided into smaller geographic areas for regulation of the power. The generating capability and the loads may vary with time and the operation of the grid must be controlled to keep the power distributed—especially the voltage and frequency—within defined characteristics.
If supply is less than demand, the frequency decreases and if supply is greater than demand, frequency increases. As a result, frequency regulation is an important part of the normal operation of modern AC electricity grids and has historically been accomplished by adjusting classic generation sources (such as hydro, gas or coal power generators) up or down to match the total demand. Independent System Operators (ISOs) are responsible for monitoring their geographic area and ensuring that the mismatch between supply and demand remains small. To do this, the ISOs monitor the total inflows and outflows from their area, the total being generated, and the total demand (or load). The ISOs use this information to determine if more or less power needs to be generated and sends a control signal to the generators under their control. As a result, the AC frequency of the entire system is managed during both regular operation (customers increasing/decreasing their demand) and emergency conditions (when a generator trips out due to a fault).
The IEEE publication “Generation Scheduling and Control”, Thomas M Athay, Proceedings of the IEEE, Vol. 75, No. 12, December 1987, provides a summary of how power generation control and frequency regulation are accomplished on most conventional AC electricity grids. The overall objective is to control the electrical power generator units so as to supply the continuously changing customer power demand in an economical manner. This task is undertaken by an Automatic Generation Control (AGC) program operating within the ISOs. Much of the associated functionality is provided by an AGC program operating within a control center computer. In very general terms, the AGC monitors the current state of the AC electricity grid (typically voltage and frequency) and outputs regulation signals for controlling each generation unit to keep the voltage and frequency within acceptable limits. An essential aspect of an interconnected system is that all available generators in the system respond to changes in frequency, via their governor speed control mechanisms and their AGC regulation signals, to bring the frequency to a new steady state, thereby balancing total system generation to total system load.
A limitation of conventional AGC regulation methods is that generation units (e.g. hydro-electric, steam-turbine, diesel etc.) frequently have a response time of several seconds, whereas total system load can vary (due to customers connecting and disconnecting loads to and from the grid) much more rapidly. Consequently, it can be very difficult for conventional AC electricity grids to track short-period variations in total system load. The result is that the frequency and voltage in the AC electricity grid will typically fluctuate in time, and the AGC system will operate to constrain these fluctuations to a predetermined range.
The publication “Energy Storage: The Missing Link in the Electricity Value Chain” by the Energy Storage Council (ESC) (2002), proposes the use of grid-connected energy storage systems which operate to supply and absorb power to and from the grid to enable improved frequency regulation in the distribution grid. As noted in the ESC paper, energy storage systems such flywheels and batteries are capable of responding to system load changes (or, equivalently, AGC regulation signals) more rapidly than conventional power generation units, which enables more accurate frequency regulation in the grid.
While the use of grid-connected energy storage systems enables improved frequency regulation relative to conventional AGC methods, further improvements are desirable.